In producing a welded steel pipe by forming a steel strip into a cylinder while continuously conveying the steel strip so that opposite ends of the steel strip are made to face each other, and welding the joint (so-called “seam”), various welding technologies are in practical use. Among such welding technologies, high-frequency resistance welding (so-called “electric resistance welding”) is a technology with which the seam can be most efficiently joined.
The electric resistance welding is a technology in which welding is conducted by heating opposite ends of a steel strip using an induction coil (performing so-called “high-frequency induction resistance heating”) or heating opposite ends of a steel strip by supplying a high frequency electric current to the opposite ends through a contact tip (performing so-called “high frequency contact resistance heating”), and applying a pressure to the molten opposite ends of the steel strip with squeeze rolls while eliminating impurities and oxides in the molten metal. The electric resistance welding has an advantage that a welded steel pipe can be efficiently produced. However, when opposite ends of a steel strip are made to face each other and melted by heating, alloying components, such as Mn, Si, and Cr, in the molten metal tend to combine with oxygen in the atmosphere to produce high melting point oxides. Such high melting point oxides remain inside the seam to generate a welding defect called penetrator.
Consequently, to suppress the generation of such high melting point oxides, a technology in which a molten metal is shielded with an inert gas (so-called “gas shielded method”) has been studied. However, since oxidation of alloying components in a molten metal may not be sufficiently prevented by the gas shielded method, generation of high melting point oxides is inevitable. Thus, the electric resistance welding is not suitable for joining alloy steels such as stainless steels containing alloying components such as Cr and Ni in large amounts (e.g., ferritic stainless steels, austenitic stainless steels, martensitic stainless steels, and dual-phase stainless steels).
Consequently, studies have been conducted on a technology in which a seam of a welded steel pipe is welded by applying a high-energy-density beam (for example, a laser beam or an electron beam). In the welding with a high-energy-density beam, generation of high melting point oxides can be prevented because a molten metal does not contact the atmosphere. However, in the welding with a high-energy-density beam, such a molted metal is formed in an extremely small region. Therefore, if a shift in the circumferential direction is generated between a position to which the high-energy-density beam is applied and a position (hereinafter referred to as “squeezing point”) at which longitudinal edges (i.e., opposite ends of a steel strip that is formed into a cylinder) of an open pipe (herein, the term “open pipe” refers to a pipe-shaped steel strip which is formed with multi-stage forming rolls and, the ends of which are not joined. Hereinafter, such a pipe-shaped steel strip is referred to as “open pipe”) to which a pressure is applied with squeeze rolls are joined to each other, the joined portion is not melted, and the edges of unmelted portions are left in a seam portion of the welded steel pipe. The portions may be removed as a welding defect. As a result, the yield rate of welded steel pipes decreases.
For example, the technology disclosed in Japanese Unexamined Patent Application Publication No. 3-291176 is a welding technology in which, as illustrated in FIG. 1 of Japanese Unexamined Patent Application Publication No. 3-291176, opposite ends 1-1 of a steel strip 1 formed into a cylinder are preheated using a first heating source 2 (i.e., high-frequency power supply for preheating edges), and then joined by applying a pressure with squeeze rolls 4 while applying a welding heat source 3 (i.e., plasma or laser). By using the first heating source 2 and the second heating source 3 in combination, the welding speed can be improved. However, in Japanese Unexamined Patent Application Publication No. 3-291176, as described in Examples, since the wall thickness t of a steel pipe to be joined is relatively small; 3 mm or less, the preheating temperature determined by the first heating source 2 is specified to be in the range of 200° C. to 600° C. When this technology is applied to welded steel pipes (e.g., UOE steel pipes, spiral steel pipes, and the like) having a large diameter and a large wall thickness t of more than 6 mm, improvement in the welding speed may not be expected at a preheating temperature of about 600° C. In addition, in Japanese Unexamined Patent Application Publication No. 3-291176, in particular, welding defects generated on the inner side of a steel pipe, the welding defects being particularly generated in welding using a high-energy-density heating source such as a laser beam may not be completely removed. Thus, this technology does not also improve the yield rate of welded steel pipes.
The technology disclosed in Japanese Patent No. 1738729 is a welding technology in which, as illustrated in FIG. 3 of Japanese Patent No. 1738729, opposite ends of a steel strip 1 are preheated using a first heating source 10 (i.e., an induction heating coil) at the upstream side (with respect to an advancing direction of a steel pipe) of a point (i.e., V-convergence point P) at which the opposite ends of the steel strip 1 that is formed into a cylinder contact each other with squeeze rolls 11a and 11b for the first time, then irradiated with a second heating source 12 (i.e., a laser beam) at the downstream side of the V-convergence point P, and joined by further pressing with squeeze rolls 13a and 13b. However, the opposite ends of the steel strip 1 are squeezed by the squeeze rolls 11a, 11b, 13a, and 13b with an amount of squeezing with which the squeeze rolls just make the opposite ends contact each other. Accordingly, in Japanese Patent No. 1738729, there may be a problem such as a difficulty of joining the opposite ends of the steel strip due to, for example, spatter generated from the irradiation point of the high-energy-density beam, or the generation of burn through or undercut in the seam. Furthermore, solidification cracking may occur in some components of a steel strip. In addition, in Japanese Patent No. 1738729, in particular, welding defects generated on the inner side of a steel pipe, the welding defects being particularly generated in welding using a high-energy-density heating source such as a laser beam may not be completely removed.
The technology disclosed in Japanese Unexamined Patent Application Publication No. 8-174249 is a welding technology in which, as illustrated in FIG. 1 of Japanese Unexamined Patent Application Publication No. 8-174249, opposite ends of a steel strip formed into a cylinder are preheated using a first heating source 6 (i.e., high-frequency induction heating device), and then joined to each other by applying a second heating source 10 (i.e., a laser beam) and simultaneously applying a pressure with squeeze rolls 4a and 4b. In this technology, the laser beam may be applied near a squeezing point 9. However, it is inevitable that the irradiation point of the laser beam and the energy density vary depending on various factors in a production line of the welded steel pipe, resulting in a problem such as meandering of the seam caused by a variation in the gap between the opposite ends 1a and 1b of the steel strip or by a shift between the irradiation point and the squeezing point 9, or the generation of a small defect called spiking caused by variations in the energy density. That is, in Japanese Unexamined Patent Application Publication No. 8-174249, in particular, welding defects generated on the inner side of a steel pipe, the welding defects being particularly generated in welding using a high-energy-density heating source such as a laser beam may not be completely removed.
Furthermore, when a welded steel pipe produced by electric resistance welding is worked (i.e., subjected to secondary working), cracks tend to be generated from, as starting points, segregation lines that are intensively exposed on the outer side and the inner side of the seam of the pipe.
Consequently, Japanese Unexamined Patent Application Publication No. 2006-150412 discloses a technology in which the seam of a welded steel pipe is locally remelted and solidified at the outer and inner sides of the pipe. This technology is usually applied to welded steel pipes produced by publicly known electric resistance welding with a specified depth of remelting, and is not necessarily applied to welded steel pipes welded with a high-energy-density beam. Accordingly, Japanese Unexamined Patent Application Publication No. 2006-150412 does not suggest a method for completely removing welding defects generated on the inner side of a steel pipe, the welding defects being particularly generated in welding using a high-energy-density heating source such as a laser beam.
Therefore, there remains room for improvement to apply the technology disclosed in Japanese Unexamined Patent Application Publication No. 2006-150412 to welded steel pipes welded with a high-energy-density beam.
An aspect of the present invention provides a welded steel pipe that is joined with a high-energy-density beam so that there are no welding defects in the seam. According to another aspect of the present invention, a method for efficiently producing the welded steel pipe is provided.